1. Field of the Invention
The present invention relates to an optical sender and a transponder capable of modulation by Differential Quadrature Phase Shift Keying, particularly, to an optical sender and a transponder used in a communication system able to send and receive optical signals at a high bit rate and modulated by Differential Quadrature Phase Shift Keying (DQPSK).
2. Description of the Related Art
In recent years and continuing, optical modulation schemes such as DPSK (Differential Phase Shift Keying) or DQPSK (Differential Quadrature Phase Shift Keying) are attracting attention as techniques allowing optical transmission at a bit rate higher than 40 Gbps.
The DQPSK modulation scheme is superior in long distance transmission, high density multiple/large capacity transmission, and in convenience of design and usage compared to other common and well-known modulation schemes such as NRZ (Non-Return-to-Zero), CS-RZ (Carrier suppressed Return-to-Zero), and RZ-DPSK (Return-to-Zero Differential Phase Shift Keying). In this specification, it is assumed that the DQPSK modulation scheme includes the RZ-DQPSK scheme for converting DQPSK signals into pulses, and the Carrier suppressed Return-to-Zero DQPSK scheme.
Below, an optical sender and an optical receiver are described briefly, which employ the DQPSK modulation scheme.
FIG. 3 is a block diagram illustrating an example of a configuration of an optical sender employing the DQPSK modulation scheme in the related art.
For details of the optical sender in FIG. 3, reference can be made to International Application's Japanese Publication No. 2004-516743, and A. H. Gnauck et al., “Spectrally Efficient (0.8 b/s/Hz) 1-Tb/s (25×42.7 Gb/s) RZ-DQPSK Transmission Over 28 100-km SSMF Spans With 7 Optical Add/Drops”, ECOC2004, PD.4.4.1.
In the optical sender shown in FIG. 3, a light source 3-1, for example, a DFB (Distributed Feedback Laser) emits a light beam, and the light beam is split into two beams. One of the two split light beams enters into a first phase modulator (PM) 3-2, and the other split light beam enters into a second phase modulator (PM) 3-3 and a phase shifter 3-4.
The phase modulators 3-2 and 3-3, being independently driven by respective modulation signals ρk and ηk generated in a precoder 3-5 from data signals Ik and Qk, selectively change phases of the respective incident light beams by 0 or π [rad]. The phase shifter 3-4 applies a phase difference of π/2 to the incident light beam propagating in a light path through the phase modulator 3-3 with respect to the incident light beam propagating in a light path through the phase modulator 3-2.
Hence, the output light beam from the light path through the phase modulator 3-2 becomes an optical signal modulated by shifting the phase of the light from the light source 3-1 by 0 or π. On the other hand, the output light beam from the light path through the phase modulator 3-3 becomes an optical signal modulated by shifting the phase of the light from the light source 3-1 by π/2 or 3π/2. By combining the output light beams from the light paths, DQPSK optical signals are generated whose phases have four different possible values of π/4, 3π/4, 5π/4, and 7π/4.
Because the bit rate of these DQPSK optical signals is twice the bit rate of the data signals Ik and Qk processed in the precoder 3-5, for example, in order to transmit the DQPSK optical signals at a bit rate of 40 Gbps, it is sufficient to drive the phase modulators (PM) 3-2 and 3-3 by using data signals at a bit rate of 20 Gbps.
If the above DQPSK optical signals are sent to an intensity modulator 3-6, which is driven by a clock signal synchronized with the data signal and having a duty ratio of 50%, and the intensity modulator 3-6 converts the DQPSK optical signals into pulses while performing Return-to-Zero processing, RZ-DQPSK optical signals are generated. Further, by increasing the duty ratio of the clock signal to 60%, Carrier suppressed Return-to-Zero DQPSK (CSRZ-DQPSK) optical signals are generated.
The precoder 3-5 performs calculations expressed by the following logical relations (2), which are obtained by expanding the following logical relations (1) and re-arranging the expansion results.
                                                                                                                                                            ρ                                                                                                                                  ⁢                          k                                                                    =                                            ⁢                                                                                                                                  (                                                                                                                          ⁢                                                                                                I                                  k                                                                ⊕                                                                                                                                  ⁢                                                                  ρ                                                                      k                                    -                                    1                                                                                                                              )                                                        _                                                    ⁢                                                      (                                                                                          I                                k                                                            ⊕                                                              η                                                                  k                                  -                                  1                                                                                                                      )                                                    ⁢                                                      (                                                                                          ρ                                                                  k                                  -                                  1                                                                                            ⊕                                                              η                                                                  k                                  -                                  1                                                                                                                      )                                                                          +                                                                                                                                                                              ⁢                                                                                                    (                                                                                                                  ⁢                                                                                          Q                                k                                                            ⊕                                                                                                                          ⁢                                                              ρ                                                                  k                                  -                                  1                                                                                                                      )                                                    _                                                ⁢                                                  (                                                                                    Q                              k                                                        ⊕                                                                                                                  ⁢                                                                                          η                                                                  k                                  -                                  1                                                                                            _                                                                                )                                                ⁢                                                  (                                                                                    ρ                                                              k                                -                                1                                                                                      ⊕                                                                                          η                                                                  k                                  -                                  1                                                                                            _                                                                                )                                                                                                                                                                                                                                                                                                          η                          k                                                =                                                ⁢                                                                                                                                            (                                                                                                      Q                                    k                                                                    ⊕                                                                      η                                                                          k                                      -                                      1                                                                                                                                      )                                                            _                                                        ⁢                                                          (                                                                                                Q                                  k                                                                ⊕                                                                  ρ                                                                      k                                    -                                    1                                                                                                                              )                                                        ⁢                                                          (                                                                                                ρ                                                                      k                                    -                                    1                                                                                                  ⊕                                                                  η                                                                      k                                    -                                    1                                                                                                                              )                                                                                +                                                                                                                                                                                                ⁢                                                                                                            (                                                                                                I                                  k                                                                ⊕                                                                  η                                                                      k                                    -                                    1                                                                                                                              )                                                        _                                                    ⁢                                                      (                                                                                          I                                k                                                            ⊕                                                                                                ρ                                                                      k                                    -                                    1                                                                                                  _                                                                                      )                                                    ⁢                                                      (                                                                                                                            ρ                                                                      k                                    -                                    1                                                                                                  _                                                            ⊕                                                              η                                                                  k                                  -                                  1                                                                                                                      )                                                                                                                                              ⁢                                                                                                      }                            (        1        )                                                                                                      ρ                  k                                =                                                                            Q                      k                                        ⁢                                          ρ                                              k                        -                        1                                                              ⁢                                          η                                              k                        -                        1                                                                              +                                                            I                      k                                        ⁢                                          ρ                                              k                        -                        1                                                              ⁢                                                                  η                                                  k                          -                          1                                                                    _                                                        +                                                                                                              I                          k                                                ⁢                                                  ρ                                                      k                            -                            1                                                                                              _                                        ⁢                                          η                                              k                        -                        1                                                                              +                                                                                    Q                        k                                            ⁢                                              ρ                                                  k                          -                          1                                                                    ⁢                                              η                                                  k                          -                          1                                                                                      _                                                                                                                                            η                  k                                =                                                                            I                      k                                        ⁢                                          ρ                                              k                        -                        1                                                              ⁢                                          η                                              k                        -                        1                                                                              +                                                                                    Q                        k                                            _                                        ⁢                                          ρ                                              k                        -                        1                                                              ⁢                                                                  η                                                  k                          -                          1                                                                    _                                                        +                                                            Q                      k                                        ⁢                                                                  ρ                                                  k                          -                          1                                                                    _                                        ⁢                                          η                                              k                        -                        1                                                                              +                                                                                    I                        k                                            ⁢                                              ρ                                                  k                          -                          1                                                                    ⁢                                              η                                                  k                          -                          1                                                                                      _                                                                                      }                            (        2        )            
In the above logical relations (1) and (2), Ik and Qk indicate logical values (1 or 0) of signals at the timing of the k-th clock pulse input to the precoder 3-5 shown in FIG. 3, and ρk and ηk indicate logical values (1 or 0) of signals at the timing of the k-th clock pulse output from the precoder 3-5. Here, the subscript k−1 indicates the logical value at the timing of the preceding one clock pulse.
In order to perform the above calculations, in the optical sender shown in FIG. 3, the output signals ρk and ηk from the precoder 3-5 are fed back to the input terminals of the precoder 3-5 through delay elements 3-6, which generate a time delay τ corresponding to one symbol.
FIG. 4 is a circuit diagram illustrating an example of a configuration of the precoder in the related art.
In FIG. 4, logical values of ρk and ηk corresponding to the preceding one symbol are fed back through delay elements D which generate a time delay τ.
FIG. 5 is a block diagram illustrating an example of a configuration of an optical receiver for demodulating the DQPSK optical signals in the related art.
For details of the optical receiver in FIG. 5, reference can be made to International Application's Japanese Publication No. 2004-516743.
In the optical receiver shown in FIG. 5, an input DQPSK optical signal is split into two optical signals, and the two split optical signals are input to delay interferometers 5-1 and 5-2, respectively.
In each of the delay interferometers 5-1 and 5-2, for example, a Mach-Zehnder light guide having two arms is formed on a silica substrate or an indium phosphide substrate, and path lengths of the two arms are designed to be different; thereby, a time delay τ corresponding to one symbol is generated between light propagating through the two arms.
In addition, it is set that interference occurs in the delay interferometer 5-1 at a delay of π/4, which is given by a phase shifter 5-3 arranged in one arm, and that interference occurs in the delay interferometer 5-2 at a delay of −π/4, which is given by a phase shifter 5-4 arranged in the other arm.
Two complementary output signals from a coupler at the output stage of the delay interferometer 5-1 are input to a differential receiving circuit 5-5 including a pair of optical detectors and an amplifier. The differential receiving circuit 5-5 generates an electrical signal Ik by demodulating an in-phase component of the DQPSK optical signal.
Similarly, two complementary output signals from a coupler at the output stage of the delay interferometer 5-2 are input to a differential receiving circuit 5-6 including a pair of optical detectors and an amplifier. The differential receiving circuit 5-6 generates an electrical signal Qk by demodulating a quadrature component of the DQPSK optical signal.
The delay interferometer 5-1 or 5-2 used in the optical receiver shown in FIG. 5 may be constructed by a light guide as described above. In addition, the delay interferometer may also be constructed, for example, by combinations of fiber fusion couplers.
Further, when demodulating optical signals by FSK (Frequency Shift Keying) or PSK (Phase Shift Keying) modulation schemes, a Mach-Zehnder type delay interferometer can be constructed by utilizing a time difference of propagation delays between two intrinsic axes of a polarization-maintaining fiber. For example, such a delay interferometer is disclosed in Japanese Laid Open Patent Application No. 5-268159.
The signals input to the optical receiver are DQPSK optical signals obtained by phase modulation according to the aforesaid modulation signals ρk and ηk, and the electrical signals output from the optical receiver are the data signals Ik and Qk before the precoder 3-5. Namely, the precoder 3-5 transmits DQPSK signals generated with modulation signals ρk and ηk obtained by performing calculations of the logical relations (2) on data signals Ik and Qk which are to be transmitted, so that the electrical signals directly output from the DQPSK optical receiver are in agreement with the data signals Ik and Qk to be transmitted.
Such a precoder is described in R. A. Griffin and A. C. Carter, “Optical Differential Quadrature Phase-Shift Key (DQPSK) for High Capacity Transmission”, Technical Digest of OFC2002, WX6.
FIG. 6 is block diagram illustrating an example of a DQPSK transponder using the above optical sender and the above optical receiver.
Shown in FIG. 6 is an example of a transponder operating at a bit rate of about 40 Gbps.
In FIG. 6, the left side is a client (user), and the right side is a network for WDM (Wavelength Division Multiplexing) transmission.
In the transponder shown in FIG. 6, data signals from the client (user) side are transmitted through an optical fiber and are received by an optical receiver 6-1 operating at a bit rate of 40 Gbps (abbreviated to be “40 G OR VSR”).
The optical receiver (40G OR VSR) 6-1 converts the optical data signals into electrical data signals and outputs 16 parallel data signals each at a bit rate of 2.5 Gbps.
A framer LSI 6-2 transforms the output signals from the optical receiver 6-1 into multiple frames, for example, SONET/SDH (Synchronous Optical NETwork/Synchronous Digital Hierarchy) or OTN (an interface for an optical transmission network recommended by ITU-T G.709). In this process, since overhead is appended to each resulting signal, the framer LSI 6-2 outputs 16 parallel data signals each at a bit rate of 2.7 Gbps as OTN or 2.5 Gbps as SONET/SDH. FIG. 6 illustrates an example of using OTN Framer in the related art. The indicated bit-rate in this figure is in the case of OTN.
A serializer (abbreviated to be SER) 6-3 converts the 16 parallel data signals at a bit rate of 2.7 Gbps as OTN or 2.5 Gbps as SONET/SDH from the framer LSI 6-2 into a serial data signal at a bit rate of 43 Gbps or 40 Gbps, respectively.
A de-multiplexer (DEMUX) 6-4 de-multiplexes the output signals from the serializer (SER) 6-3 at a ratio of 1 to 2, and generates two parallel data signals Ik and Qk each at a bit rate of 21.5 Gbps as OTN or 20 Gbps as SONET/SDH.
The output signals from the de-multiplexer (DEMUX) 6-4 are input to a DQPSK precoder 6-5, which has the same configuration as described above. The DQPSK precoder 6-5 outputs data signals ρk and ηk obtained by calculations of the logical relations (2).
The output signals ρk and ηk are input to a DQPSK optical sender 6-6 (abbreviated to be “40G OS DQPSK”), and the optical sender 6-6 sends optical signals at a bit rate of about 43 Gbps as OTN or 40 Gbps as SONET/SDH to the network.
Meanwhile, the optical signals at 43 or 40 Gbps from the network are received by a DQPSK optical receiver 6-7 (abbreviated to be “40G OS DQPSK”). The DQPSK optical receiver 6-7 outputs data signals Ik and Qk each at a bit rate of about 21.5 or 20 Gbps.
A multiplexer (MUX) 6-8 multiplexes the data signals Ik and Qk from the DQPSK optical receiver 6-7 at a ratio of 2 to 1 to convert the data signals Ik and Qk into a serial data signal at a bit rate of about 43 or 40 Gbps.
The output signals at 43 or 40 Gbps from the multiplexer (MUX) 6-8 are input to a De-serializer (DES) 6-9. The De-serializer (DES) 6-9 converts the serial data signal at 43 or 40 Gbps into 16 parallel data signals each at 2.7 or 2.5 Gbps.
The framer LSI 6-2 extracts and outputs data signals of 16 channels from the OTN or SONET/SDH multi-frame signals each at about 2.5 Gbps.
The 16 parallel data signals at about 2.5 Gbps are sent to an optical sender 6-10 (abbreviated to be “40G OS VSR”).
The optical sender (40G OS VSR) 6-10 converts the 16 parallel data signals into a serial data signal at about 40 Gbps, and sends optical signals of the serial data at about 40 Gbps to the client through the optical fiber.
However, during the calculations by the logical circuit including multiple stages as shown in FIG. 4, in the related art, in the DQPSK optical sender used for optical transmission at a bit rate higher than 40 Gbps as described above, it is required that the precoder perform the calculations and output the data signals at a very fast clock speed, specifically, a bit rate of about 20 Gbps, which is comparable to the bit rate of the transmission signals. The DQPSK precoder is not able to perform the logical calculations at a low clock speed.